A NEW APPROACH TO CONSTRUCTION OF UNDERGROUND STATIONS AND
LAUNCH BOXES
James C. Thomson C.Eng
Jacked Structures SA
Background
The use of jacking methods to install underground pipes was first recorded more than a 100
years ago.
It was some 50 years ago that a new wave of development in mainland Europe and UK that
rapidly established pipe jacking as an effective method of direct installation of sewers and
other utilities. Initially excavation was by hand from within open face shields. In Japan the
development of remote control slurry pressure balance machines in combination with jacked
installation of pipes allowed non-man entry, smaller diameters and use in difficult ground
conditions .
Applications developed including larger diameters tubes of 3 and 4m diameter jacked into
place beneath existing highways and railway infrastructure for pedestrian subways. Smaller
rectangular sections were also jacked in below rail tracks and roads to form culverts and
pedestrian subways.
There was an increasing need to install larger structures, such as road underpasses under rail
tracks and highways with minimal disruption which encouraged the author to make a number
of developments when working for Cementation/Trafalgar House contractors in the nineteen
sixties and seventies.
A patented development was modular jacking where precast box sections are jacked into
position one above the other to form composite abutments and piers with the deck installed
during a short possession of the railway or highway. An example of this method is the
Wandsworth Bridge underpass which was installed in 1969 below 7 busy rail tracks coming
into central London. The concept is illustrated in Figure 1.
Figure 1 Modular System
The Brent Cross Tunnels installed in 1974 under the busy A41 arterial route in north London
was one of the earliest full size jacked box installations for road underpasses with box
widths of 9.5m and height of 7m the tunnel units were cast on site adjacent to the existing
highway embankments The patented friction reducing drag sheets were used and proved to be
a key to jacking large box sections with shallow cover. The concept of box jacking is
illustrated in Figure 2
Fig 2. The Box Jacking Concept
Both box jacking and modular jacking has been employed on numerous jobs in UK and
internationally. These techniques are still widely used today.
Jacked Arch and Jacked Deck Development
The modular and the jacked box have some shortcomings in certain circumstances.
Jacked boxes:
 Require a large site footprint for the construction of the box which can involve large
expensive launch pits when the underpass is deep below ground level.
 In unstable soils face stability can be a concern in large boxes with exposed faces 8m
or more high
 Jacking forces and reaction arrangements are required to accommodate high loads as
boxes get larger in cross section and drive length.
 It can be sometimes difficult to control line and level; in harder conditions at the
invert there is a tendency to climb as it is difficult to trim accurately. In soft invert
conditions it is difficult to stop a tendency to settle.
 With large boxes steerable shields are not effective
Modular Jacking although overcoming several of the shortcomings of large boxes has the
major disadvantage that an abnormal possession of the highway or railway is required to
install the deck.
Considering these drawbacks the author has developed patent protected innovations of the
Jacked Arch and the Jacked Deck. These provides safer, faster and less disruptive ways of
installing large underground structures of greater clear spans over longer lengths.
Both methods have in common the use of a slide path which is installed within a jacked box
or tunnel. This ensures that the slide track is fully prepared ahead of installation and the
structure is installed with lower jacking forces and to close accuracy.
The Jacked Arch
Figure 3 Jacked Arch Concept
The process consists of the following stages :
Stage 1. The installation of man-entry foundation boxes designed for the foundation loads
and the soil conditions. These are jacked in to position.
Stage 2. After driving the boxes a guide path for the arch is pre-set manually inside the
boxes
Stage 3. The main structure is formed from pre-cast arch sections (2 or 3 pin) which are
jacked in behind a shield to form the canopy structure. The leading arch unit is fitted with
a compartmentalized raking shield.
The base of the arch sections, as they are driven forward, are located and slide in the guide
path (installed at Stage 2). The foundation boxes have roof sections which can be removed
progressively from inside the arch shield to expose the guide path.
Stage 4. Joint sealing, internal finishes and a floor are installed appropriate to use.
A number of variations are possible including elements of the modular system to
create vertical walls combined with flat arches. It is also possible to use circular
foundation tubes rather than boxes. Such a configuration could allow the use of
pressure balance tbm’s where ground conditions and water tables are a concern.
The Jacked Deck
Figure 4 The Jacked Deck Concept
The jacked deck is appropriate for spans of 10m or greater and figure 4 shows alternative
options for creating the slide path and foundation.
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The outer abutments are installed using modular jacking and the slide path is installed
in the top unit
The central pier support is formed by jacking a high level box and piling from inside
the box.
The deck can be constructed from precast concrete beam sections or multiple cast on site units
and is designed with a downstand providing an overall minimum depth of 2.5 m or more. It is
possible to install on the leading edge of the deck a suitable compartmentalized shield in
which general labour and machines can excavate the face. The preinstalled piers and
abutments support and confine the earth allowing excavation to a greater depth below the
deck units providing access for machines and men and spoil disposal
This concept is being employed in the construction of a key road underpass below rail tracks
by the Volker Fitzpatrick Hochtief JV as part of the £62 million East Kent Access project.
With a 19.950 m clear internal span and a drive length of 125m it is one of the largest and
longest underpasses ever constructed beneath an operational railway In this case the
contractor opted for installing 3.4m segment access tunnels from which piles are driven to
provide the abutments and for the installation of the slide path. This is illustrated at Fig 5
The detailed design was developed by Atkins with Jacked Structures SA. providing specialist
technical support on the installation.
Figure 5 Cliffsend Jacked Deck Underpass (VolkerFitzpatrick Hochtief JV)
Application of Jacked Arch and Jacked Deck to Underground Stations
Having established some background and an example of the jacked deck concept the second
part of this presentation is concerned with the application of these concepts to the construction
of underground stations , TBM launch boxes and similar large underground structures.
The jacked arch and jacked deck technologies have a capability for installing underground
stations.
All the options described provide for a slide track within a foundation, access or wall box
along which the arch or deck units are jacked as excavation proceeds. One method is based
on the modular construction of piers and abutments with the top wall box providing the
location of the slide track. An alternative is the jacked access box from which piled
foundations can provide abutment walls or columns as well as the slide track. A variant is to
use a circular tunneled access instead of the jacked box which could be used where ground
conditions warrant. Where ground conditions require treatment these same access boxes or
tunnels can be used for pre-stabilization of the ground prior to installing the jacked arches or
deck beams.
The following sketches are illustrative of the method and both the dimensions and the
configurations can be varied to meet the specific needs
In most cases, because of restricted space at the working site, it is anticipated that precast box
units, arch sections and deck beams will be factory cast and transported to site. Regulations
vary from one country to another on the maximum size of unit that can be transported on
public roads. For larger diameter arches it would be necessary to precast these in two
sections, for example a 3 pin arch or for the two elements to be joined at site to form a 2 pin
arch
It is envisaged that the techniques set out below would have application wherever surface
disruption is to be avoided. Where structures have to be constructed with a few meters of
cover these techniques, particularly the jacked deck, would be beneficial as they are
engineered to minimize any surface settlement. Equally large deep structures would benefit as
the area of exposed soil face within the shield is minimized and a full structural lining is
provided at each stage of excavation.
The sketches show a range of alternative abutment and pier configurations and can be
modified depending on the needs and the site conditions. The illustrative graphics are based
on a 12m width between platforms which would accommodate twin tracks with wider trains
than London underground requirements. London Underground stations are typically 6.2-6.7m
diameter to accommodate a single track with platform. A central platform requires a
minimum 6m plus two tracks therefore a 14m arch would be sufficient.
Figure 6 A station configuration for twin tracks and side platforms based on driving
foundation boxes and full arch sections jacked in on slide tracks
Figure 7 An alternative station configuration for twin tracks and side platforms based
on driving foundation boxes. wall boxes and top arch sections jacked in on slide tracks
Figure 8 An alternative station configuration for twin tracks and side platforms based
on piling from jacked high level access boxes deck sections jacked in on slide tracks
Figure 9 An alternative station configuration for four tracks and side platforms based
on piling from jacked high level access boxes deck sections jacked in on slide tracks
Figure 10 shows a design for a complete station and is based on driving in both directions
from a central excavation which would form the access concourse. In this example a high
level box with piling and relative flat arch with down-stands is shown. But the Jacked Arch
and Jacked Deck options shown in previous graphics could equally been used.
The length of station is determined by platform length required to accommodate the number
of carriages. A platform length of 140m is shown which will accommodate up to 8 carriages.
Longer lengths would be possible.
Figure 10 An underground station based on abutments formed from piles installed from
a high level box and a jacked arch
The jacked arch and jacked deck approaches offer alternatives to existing methods of
installation. As full structural lining is installed at every stage as excavation proceeds this can
be an advantage in risk minimization.
Application of Jacked Deck to Launch Boxes
Launch boxes are constructed to provide an underground box from which TBMS are launched
to construct the running tunnels. Thereafter they provide the basic structure within which the
permanent station is constructed.
Typically these will be 22-24m span and around 200m long. The height will be determined by
the diameter of tunnel to be driven. For a twin track tunnel this could be 6/ 7m diameter with
internal depth of box up to 8-10m.
Launch boxes could be driven from a central access shaft in both directions using the jacked
deck method to give the clear spans required.
The various geometric alternative configurations for jacked arch and deck concepts described
and illustrated above would be modified to provide the required span, length and height.
Application to Other Large Underground Structures
Underground Parking
Both the jacked arch and jacked deck could used to create underground parking below streets,
buildings, parks etc.
We envisage that a series of interconnecting bays would be created either using an arch or a
jacked deck concept.
A clear span of 10-12m for each bay would typically be required to provide parking bays
either side of a central access road
Underground Storage Caverns
There are a number of developers and industries that have a need to create storage space in
locations where open cut would be too disruptive or not cost effective. These could be
constructed by the jacked arch and deck techniques.
Advantages of the Jacked Approach
The application of jacked arch and deck methods provides safer, faster and less disruptive
installation of large underground structures including underground stations and launch boxes.
Specifically:
 By breaking down the process into elements reduces the exposed soil face at any point
and reduces jacking forces and reaction arrangements.
 Continuous support of the excavation with a full structural lining as installation proceeds
minimises the possibility of soil collapse
 The risk of injury or death to workers is virtually eliminated
 Any surface settlement is minimal and adjacent buildings, property and the public are
protected
 Use of precast units allows higher quality construction
 A wide range of geometry and configuration can be tailored to each project
 Installation to very close line and level tolerances
 The methods are applicable to constructing both shallow and deep structures
 Jacked arch and jacked deck structures can provide greater clear spans over much longer
drive lengths than existing jacking techniques.
 Jacking precast elements to form the underground structure can be undertaken from
relatively small launch platform excavations.
James C. Thomson C.Eng.
Jacked Structures SA
james.thomson@jackedstructures.com